In one sentence
The share of your weight that is water, and that share in kilograms, which is the one thing a bioimpedance scale measures directly and the thing behind most of the day-to-day wobble in every other number.
Why it matters
Water is where the noise lives. About 73 percent of lean tissue is water (Kyle 2004), so every number the scale derives from lean tissue, body fat included, moves when your water does. Watching body water beside weight and body fat tells you which of the day's changes are hydration and which might be something else. It is also the quantity bioimpedance was built to measure: the current the scale sends passes through water, and total body water is what the impedance reading estimates best.
What moves it
Drinking and sweating, obviously; carbohydrate, because glycogen is stored with about three times its weight in water (Fernández-Elías 2015); salt; alcohol, which sheds water first and holds it afterward; heat and saunas; a hard session; the menstrual cycle; and time of day. Typical values sit between 50 and 65 percent of body weight, higher in lean people and lower in people carrying more fat, because fat holds little water.
How to measure it well
The scale's routine, with the morning-after-the-toilet rule mattering most here.
How Vitality Tracker uses it
Stored from the full scale reading, with the extracellular and intracellular split and their ratio where the scale reports them; charted beside weight and body fat so a water day is visible as one. Nothing scores on it.
How far to trust the number
Estimate; good at following itself on one scale, which is all it needs to do.
Sources
- Kyle UG, et al. Bioelectrical impedance analysis, part I. Clin Nutr 2004;23:1226–43. doi:10.1016/j.clnu.2004.06.004
- Fernández-Elías VE, et al. Relationship between muscle water and glycogen recovery after prolonged exercise in the heat in humans. Eur J Appl Physiol 2015;115:1919–26. doi:10.1007/s00421-015-3175-z.